Vibration reduction in mechanical systems is important for a number of reasons including safety and energy efficiency of the systems. Particularly, vibration in various transportation systems is directly related to ride quality and safety of passengers, and, thus, should be reduced. For example, vertical vibration in vehicles can be controlled by active or passive vibration reduction systems, which are generally referred as suspension systems. Similarly, the vibration induced during an operation of an elevator system can be reduced.
The elevator system typically includes a car, a frame, a roller guide assembly, and guide rails. The roller guides act as a suspension system to reduce the vibration of the elevator car. The car and roller guides are mounted on the frame. The car and frame move along the guide rail as constrained by the guide rollers. When the elevator moves sufficiently fast, level variation or winding of the guide rails can induce significant lateral vibration in the frame and the car.
The vibration induced by, e.g., deformation of the guide rail, can be reduced by various types of the suspension system. Generally, there are passive, semi-active, and active types of the suspension systems. The passive suspension system has undesirable ride quality. The active suspension systems use separate actuators that can exert an independent force on the suspension to improve the riding and can provide desirable performance for reducing the vibration. The drawbacks of the active suspension system are high cost, added complication and mass, and the need for maintenance.
The semi-active suspension systems provide a better trade-off between the cost of the system and its performance. A semi-active actuator allows for the adjustment of parameters, such as viscous damping coefficient or stiffness, and can be used to reduce the vibration, and is reliable because such actuator only dissipates energy.
For example, one system, described in U.S. Pat. No. 5,289,902, reduces the lateral vibration of elevators using semi-active actuators, such as a hydraulic actuator. That system adjusts the damping coefficient of the actuator by controlling a movable orifice lever in a solenoid. However, because of the absence of control mechanism, the achievable performance might be limited, see also U.S. patent publication 2009/0294222. In another example, a vibration damper is used to reduce the axial and rotational vibrations of an automotive steering system, see U.S. Pat. No. 6,752,425. The vibration damper may be activated or deactivated by a controller by comparing a signal value from a steering wheel vibration sensor to a predetermined threshold value. Using semi-active actuators with variable stiffness is also described in U.S. patent publications Ser. No. 10/574,653, and in U.S. Pat. No. 7,543,686.
U.S. Pat. No. 5,712,783 discloses a vibration reduction method according to the skyhook damping to control an automotive load-leveling suspension, i.e. switching ON and OFF of the semi-active. This method uses relative position sensors to obtain the relative velocity, which is difficult and leads to unnecessary system cost.
Conventional semi-active vibration reduction requires the measurement of the relative velocities between the ends of semi-active actuators. The relative velocity is critical to determine the time to switch the semi-active actuators ON and OFF, which is directly associated with the vibration reduction performance.
Unfortunately, the measurement of the relative velocity adds to the cost, and reduces the reliability of the systems. Also, the measurement of the relative velocity is difficult, and sometimes impossible.